I Have to build a Dashboard to control the ETL process, the ETL Procedures have already been created and are stored in the database. I want the user to be able to execute some procedures from an OBIEE dashboard. how do you implement it?

Evaluate function is used to call either functions or Stored procedures.
But, as per your requirement .. you wanted to fire the stored procedure from front end report i.e. execute the stored procedure.

And, all the reports which we build is 'Select' for simple. But, not Execute (As per my knowledge)
But, to execute a procedure - you need to run Execute command.

Can, you check for the following options:

1. connection script in connection pool
2. Any Java scripts .. which helps you to create a check box or button which by clicking executes the back end stored procedure. (And, copy that particular code into Dashboard object Text).